The present invention relates to the early detection of event conditions which include a change in the amount of a reactive gas, such as hydrogen chloride (HCl) or another of the halogen acids, in the event environment. An increase in such a gaseous constituent is typical, for example, when a fire occurs amidst electrical and electronic equipment where the use of polyvinyl chloride (PVC) wiring insulation and halogenated fire retarding adjuncts are prevalent, as in a telecommunications central office. In such a fire, these gases alone may be to a great extent responsible for significant equipment damage due to their corrosive effect. It is therefore essential that an effective warning system for such locations be able to readily detect the presence of these gases, since they will often accumulate and initiate equipment damage well prior to any onset of and at some distance from the area of actual conflagration.
The various conventional fire detectors which are currently in use generally respond to an increase in the concentration of particulate matter generated by fire conditions. The size and amount of this particulate matter that is evolved during pyrolysis or combustion of most fuel materials will depend to a great extent upon the conditions of the fire as well as on the chemical makeup of the fuel. As a result, the types of detectors and their methods of measuring the particles generated by fires will vary widely.
For example, ionization detectors employ a radioactive source, usually an alpha emitter, which causes ionization of air in a sensing chamber. When ionized, the charged molecules are accelerated toward a polarized electrode, thereby causing the flow of an electrical current. Smoke or other particulate combustion products entering the chamber cause a notable change in the ionization current. Owing to this manner of operation, ionization detectors are more responsive to small particles, typically less than 1 micrometer, and are thus effective in the presence of flaming fires.
Photoelectric detectors, on the other hand, are typically based on the effect of the combustion particles in causing the scattering of light which is normally transmitted across a detection chamber. The increase in intensity of diverted or scattered light is measured by a photodetector placed at an angle from the transmitted light beam. Since large, non-light-absorbing particles scatter light more effectively, these detectors are more responsive to such large particulate matter, typically of a diameter greater than about 1 micrometer. As a result, these detectors tend to be more effective with smoldering fires or fires in materials which burn inefficiently, since under these conditions large particles are predominantly emitted during pyrolysis or combustion.
Optical beam detectors are somewhat similar, but are based on a reduction in the intensity of light emitted from a source and measured by a photodetector located in the light path. This path is often quite long, since the source and detector are commonly mounted on opposite walls of a room under surveillance. Smoke or other combustion products moving into the light path cause either an absorption or obscuration reduction in the intensity of light reaching the photodetector.
Notably lacking among these devices, however, are systems employing chemical detection methods, that is, systems in which a chemical sensor is used to detect an increase in the ambient concentration of gaseous constituents, such as CO, CO.sub.2, HCl, HBr, and the like. Although studies of telecommunications central office fires have established that a primary cause of extensive electrical equipment damage is the corrosive effect of halogen acid gases generated by thermally decomposing PVC cable and wire insulation and treated circuit boards, few chemical fire detectors are commercially available which would aid in the mitigation of such damage. A lack of commercial systems of this type is, for the most part, apparently due to the poor specificity or reliability of existing chemical sensors or sensing systems, as well as the unfavorable economics for their widespread use.
A number of gas analysis techniques are known by which a reactive gas such as HCl might be identified, but few of these have found practical application in fire detection devices. Gas trapping combined with coulometric, titrimetric, or colorimetric analysis, for example, has limited specificity and involves extended procedures which defeat near real-time measurement and response. Infrared absorption spectroscopy and indirect chemiluminescence have shown some improved selectivity and response time; however, these capabilities are still significantly short of practical levels and the costs of implementing and maintaining fire detection systems incorporating these sensor types remain prohibitive.
There are currently some commercially available systems for detecting HCl, for example, which employ sensors based upon electrochemical methods of analysis. These amperometric, coulometric, and potentiometric systems generally suffer, however, from a lack of chemical specificity and often require considerable maintenance. The sensing elements are also particularly expensive and would therefore not be useful for implementation in economical, multi-detector systems. Other available systems rely upon significantly less expensive high temperature semiconductor sensors; however, these detectors suffer greatly from a lack of chemical specificity and often have extremely demanding electrical power requirements.
One area of gas-sensing technology that has exhibited significant promise, however, for use in an economical system for selective detection of reactive gases such as HCl and other halogen acid gases is that of the piezoelectric quartz crystal microbalance. Piezoelectric resonators are available at low cost and their chemical specificity can be controlled by coating with a compound specifically responsive to the presence of HCl, for example. Such use of a zinc-coated crystal resonator microbalance has been described by Neuburger (G. G. Neuburger, Anal. Chem., Vol. 61, No. 14, pp. 1559-1563, July 1989, incorporated herein by reference, and has been particularly successful in the early detection of fires affecting PVC and halogenated fire retardant materials. A practical reactive gas detection system may be based upon this technology, but requires adaptive control for reliable response and economical performance.